Three examples of such conventional pulse height analyzer are to be cited for description of a pulse height analyzer provided in a radiation detector. Here, a radiation pulse is obtained by converting emission due to interaction of incident gamma rays or X-rays on a scintillator into a current pulse by a photoelectric transducer, such as a PMT (Photomultiplier tube) and a photodiode, and further converting the current pulse into a voltage pulse via a current-to-voltage conversion circuit. Although the radiation pulses illustrated each differ in event, they are to be illustrated for convenience as having been detected at the same time.
The first technique is as below. That is, waveform shaping is performed on a radiation pulse having a pulse wavelength, as the sum of a rise time and a fall time of a pulse, of a few tens of nanoseconds with use of a filter (CR integration), until the pulse has a wavelength of a few microseconds. Then sample hold is performed on the radiation pulse when a given period of time (e.g., 500 nanoseconds) elapses after the radiation pulse on which the waveform shaping has been performed exceeds a threshold voltage. Then an analog-to-digital conversion is once performed on the peak value of the threshold voltage, whereby a pulse height is obtained (hereinafter, called a filter integration method.)
The second technique is as below. That is, waveform shaping is performed on a radiation pulse having a pulse wavelength of a few tens of nanoseconds with use of a filter (CR integration) until the pulse has a wavelength of a few hundreds nanoseconds. Then an analog-to-digital conversion is performed for eight times at intervals of a given period of time (e.g., 20 nanoseconds) after the radiation pulse on which the waveform shaping has been performed exceeds a threshold voltage. Then all pulse heights obtained through the analog-to-digital conversion are added to determine a pulse height of the radiation pulse, whereby a pulse height of the radiation pulse is obtained (hereinafter, called a digital integration method.)
The third technique is as follows. That is, waveform shaping is performed on a radiation pulse having a pulse wavelength of a few tens of nanoseconds with use of a filter (CR integration) until the pulse has a wavelength of a few hundreds nanoseconds. Then a pulse-time width is determined from when the radiation pulse on which waveform shaping has been performed exceeds a threshold voltage until it returns to the same threshold voltage, whereby a pulse height is obtained (hereinafter, called TOT (Time Over Threshold) method.) See, for example,
Hiroyuki Takahashi, Takeshi Fujiwara, Kenji Shimazoe “A prospect of PET apparatus development (26); Application of Time over threshold method in front-end signal processing” NIRS-R (National Inst. of Radiological Sciences) National Institute of Radiological Sciences, Apr. 8, 2009. http://jglobal.jst.go.jp/public/20090422/200902256578352763
The conventional techniques, however, have the following drawbacks. Specifically, in the digital integration method, an analog-to-digital conversion is performed on the peak value of a radiation pulse for determining a pulse height. Accordingly, an analog-to-digital converter with high precision is required. Moreover, in the digital integration method, an analog-to-digital conversion is performed for eight times at intervals of 20 nanoseconds, for example, to determine each pulse height. Then all pulse heights are added to determine a pulse height. Accordingly, an analog-to-digital converter with an extremely high-speed is required. Both the analog-to-digital converters are expensive. Consequently, the number of analog-to-digital converters increases for use of individual analog-to-digital conversion of the radiation pulse successively outputted from the radiation detector that is formed of many radiation detecting elements, which results in huge costs. Moreover, since the processing and the control circuit are complicated, these techniques are not suitable for multi-channel applications.
In the TOT method, a pulse time width is determined, whereby a pulse height is determined. Thus, an analog-to-digital converter is not needed. Accordingly, the method may achieve an extremely simple configuration, and is also suitable for multi-channel applications. On the other hand, as illustrated in FIG. 10, linearity of the pulse time width relative to the pulse height becomes lower as the pulse height becomes higher. For instance, FIG. 11A illustrates radiation pulses Pa4, Pa5, and Pa6 after waveform-shaped. Here, a pulse time width of the radiation pulse Pa4 (a pulse time width from when the pulse exceeds the threshold voltage vth until returns to the threshold voltage vth) is Wtot4. A pulse time width of the radiation pulse Pa5 is Wtot5, and a pulse time width of the radiation pulse Pa6 is Wtot6. In comparison of pulses of high pulse heights such as the radiation pulses Pa4, Pa5, and Pa6, there is not so much difference between the pulse time widths Wtot4, Wtot5, Wtot6, rather than the pulse heights. In other words, the pulse height to be determined has a reduced accuracy. Accordingly, the situation as illustrated in FIG. 11B may occur. That is, a radiation pulse Pah of a high pulse height (e.g., radiation pulse Pa4, Pa5) that is originally out of an energy window EW enters in error into the energy window EW (e.g., the energy window EW containing the radiation pulse Pa6.) As a result, scattered components enter into an energy spectrum Pat in the energy window EW, whereby an energy spectrum Paf with low accuracy is generated. Consequently, the lower accuracy of the pulse height cannot be improved even when correction is performed using a look-up table. Thus, the problem still remains that energy resolution gets worse.
This invention has been made having regard to the state of the art noted above, and its object is to provide a pulse height analyzer that allows improved linearity of a pulse height relative to a pulse time width as well as enhanced energy resolution.